Scaling down optical elements is essential for making compact optical systems. Accordingly, planar and thin meta-structures are desirable for fabrication as well as for integration simplicity. Currently, dielectric-based refraction microlenses typically used in optical systems have a non-uniform design, with a thickness of tens of micrometers. Gradient-index lenses are planar; however, their thicknesses are about an order of magnitude larger than the wavelength of incident light and the fabrication process is not straightforward. Classical diffraction lenses, such as Fresnel lenses, are planar and thin; however, they cannot control the phases precisely because the position of each Fresnel ring is determined by the geometry. Phase-controlled diffractive microlenses present a combination of a diffraction lens and a refraction lens, and have been studied both experimentally and numerically, but their structures are not planar.
Recently, there has formed a growing interest in planar, sub-wavelength-thick metallic lenses in the optical range. Different kinds of metallic lenses have been proposed and experimentally demonstrated, such as the superlens, the hyperlens, surface-plasmon focusing lenses, and superoscillation-based lenses. Some of the metallic lenses result in focus spots on surfaces, while others focus in far field. Nanoslit lenses are one of the planar metallic lenses which are made of arrays of subwavelength slits milled into metallic films. Each slit width is varied to change the mode index of the single-mode light propagating through it, such that light transmitted through different slits experience different phase delays. Hence, for example, by using a symmetric array of slits with decaying phase shifts relative to the optical axis, it is possible to arrange a concave phase front, and focus a linearly polarized transmitted light. The very first designs of nanoslit metal lenses were first modeled numerically and then demonstrated experimentally.
The early work in this area dealt with plasmonic mode propagation through metallic slits. The inventors of the present invention have also recently shown that photonic modes can be efficiently used to introduce phase delay; see, for example, Ishii, S. et al. Opt. Lett. 2011, 36, (4), 451-453, incorporated in its entirety into the present disclosure. The use of either plasmonic mode or a photonic mode enable the ability to design polarization-selective nanoslit lenses whose focusing properties become either a convex (light-focusing) or concave (light-diverging) lens depending on the incident linear polarizations. The focusing properties of the nanoslit lenses can be additionally controlled by incorporating liquid crystals inside the slits.
Although nanoslit lenses are indeed planar and quite thin, an important drawback of the nanoslit design is its polarization-dependence. Moreover, as a nanoslit lens is focusing light into a narrow strip, it does not allow for high-intensity confinement of light into a wavelength-size circular spot. For some applications, the above features hinder the reduction of nanoslit lenses to practice. Thus, there is a need for a novel thin and planar optical device that addresses the drawbacks identified above.